1. Chapter 1: Introduction and State of the art

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Instituto Tecnológico y de Estudios Superiores de Monterrey

Campus Monterrey

School of Engineering and Sciences

Investigation of the adhesion of MWCNTs on Inconel 718 metallic powders for their potential use in the development of aerospace applications.

A thesis presented by Engr. Eduardo Bernales Soto

Advisor:

Dr. Alex Elías Zúñiga Co-Advisor:

Dr. Nicolás Antonio Ulloa Castillo

Submitted to the

School of Engineering and Sciences

in partial fulfillment of the requirements for the degree of Master of Science

In

Nanotechnology

Monterrey Nuevo León, December 10^th, 2019

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Declaration of Authorship

I, Eduardo Bernales Soto, declare that this thesis titled, “Investigation of the adhesion of MWCNTs on Inconel 718 metallic powders for their potential use in the development of aerospace applications” and the work presented in it are my own. I confirm that:

• This work was done wholly or mainly while in candidature for a research degree at this University.

• Where any part of this thesis has previously been submitted for a degree or any other qualification at this University or any other institution, this has been clearly stated.

• Where I have consulted the published work of others, this is always clearly attributed.

• Where I have quoted from the work of others, the source is always given.

With the exception of such quotations, this thesis is entirely my own work.

• I have acknowledged all main sources of help.

• Where the thesis is based on work done by myself jointly with others, I have made clear exactly what was done by others and what I have contributed myself.

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Dedication

For my mother, grandmother, family and Salma, the love of my life. Thanks to them I had the courage to pass my limits and have another vision of my life. For all your unconditional trust, support, patience, and encouragement: Thank you!

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Acknowledgements

I would like to express my deepest gratitude to Dr. Alex Elías Zúñiga for all the support he gave me since I entered to study this master’s degree and to my co- advisor Dr. Nicolás Ulloa Castillo who from the beginning was there providing me with the necessary help to develop my skills .

Finally, all of these wouldn’t be possible without the sponsorship granted by Tecnológico de Monterrey through the Research Group of Nanotechnology for Devices Design, and by the Consejo Nacional de Ciencia y Tecnología de México (Conacyt), Project Numbers 242269, 255837, 296176, National Lab in Additive Manufacturing, 3D Digitizing and Computed Tomography (MADiT) LN299129 and FODECYT-296176. These institutions believed in my potential as a professional and researcher.

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Investigation of the adhesion of MWCNTs on Inconel 718 metallic powders for their potential use in the development of aerospace

applications by

Eduardo Bernales Soto

Abstract

This research work focuses on studying the adhesion of Multi-Walled Carbon Nanotubes (MWCNT) to Inconel 718 metallic powders to enhance its physical properties. The adhesion of functionalized MWCNTs to the surface of the Inconel 718 metallic powders was carried out by drip incorporation. Experimental results by X-ray Diffraction (XRD), Fourier transform infrared (FT-IR), Thermogravimetric Analysis (TGA), and Scanning Electron Microscopy (SEM) provide evidence that the process followed to adhere the MWCNTs to the metallic powders was successfully achieved.

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List of figures.

Fig 1 Ferrous Microstructure example. Adapted from [20] _______________________________________ 14 Fig 2 Microstructure of Inconel 718 _________________________________________________________ 17 Fig 3 Body Centered Tetragonal atomic arrangement. __________________________________________ 22 Fig 4 Face Center Face atomic arrangement __________________________________________________ 22 Fig 5 Internal parts of a turbine. ___________________________________________________________ 24 Fig 6 Pyramidalization angles present in different molecular structures [51]. ________________________ 26 Fig 7 Graphene leaves a) Pristina, b) Hydrogenated and c) Stressed _______________________________ 27 Fig 8 Representation of MWCNTs. __________________________________________________________ 29 Fig 9 Oxidation of alkenes by potassium permanganate (KmnO4). ________________________________ 31 Fig 10 Oxidation of alkenes by ozone followed by hydrogen peroxide. ______________________________ 32 Fig 11 Oxidation of alkenes by hydration of alkenes. ___________________________________________ 32 Fig 12 Formation of covalent bonds C-F. _____________________________________________________ 33 Fig 13 Scheme of chemical modification of carbon nanotubes by acid treatment for anchoring carboxyl groups – adapted from [86]. ______________________________________________________________ 33 Fig 14 Fe3O4 coated with citric acid – Adapted from [88]. _______________________________________ 34 Fig 15 Representation of citric acid coating metal oxide surface, adapted from [92]. __________________ 35 Fig 16 Fabrication procedures for CNTs/Al composite powder – Adapted from [45]. ___________________ 36 Fig 17 FE-SEM of CNT/Al powders. (a) CNT clusters in untreated Al powders, (b)-(c) uniform adsorption of CNTs achieved by Al-PVA powders. Adapted from [45]. _________________________________________ 36 Fig 18 (a) A schematic of GOCIP synthesis process and (b) the possibility of GOCIP bonding mechanism – Adapted from [95]. ______________________________________________________________________ 37 Fig 19 Particle size distributions of nickel powders for different milling times, adapted from [98]. ________ 38 Fig 20 Evolution of morphology in aluminum matrix. Adapted from [100]. __________________________ 39 Fig 21 CNTs in aluminum matrix. Adapted from [100]. __________________________________________ 39 Fig 22 Inconel 718 Powder. _______________________________________________________________ 41 Fig 23 MWCNTs structure. ________________________________________________________________ 41 Fig 24 Citric Acid (𝐶6𝐻8𝑂7). ______________________________________________________________ 42 Fig 25 Parafilm. ________________________________________________________________________ 42 Fig 26 Acids used A) Sulfuric Acid (H2SO4), B) Nitric Acid (HNO3) __________________________________ 42 Fig 27 Characterization by SEM of Inconel 718 powder as received. _______________________________ 43 Fig 28 Characterization by EDS to know chemical composition of Inconel 718. _______________________ 44 Fig 29 Characterization by XRD of Inconel 718 powder as received. ________________________________ 44 Fig 30 Characterization by FTIR of Inconel 718 as received. ______________________________________ 45 Fig 31 Characterization by SEM of MWCNTs as received. ________________________________________ 46 Fig 32 Characterization by XRD of MWCNTs as received. ________________________________________ 46 Fig 33 Characterization by TGA of MWCNTs as received. ________________________________________ 47 Fig 34 Characterization by TGA of Citric Acid as received. _______________________________________ 47 Fig 35 Characterization by FTIR of Citric Acid as received. _______________________________________ 48 Fig 36 Diagram of the different process routes. _______________________________________________ 49 Fig 37 Weighted Inconel 718 and balls in closed containers. _____________________________________ 51 Fig 38 Sealed Jar with Inconel 718 and the balls inside. _________________________________________ 52 Fig 39 Fixed Jar in the mill. ________________________________________________________________ 53 Fig 40 Moment just before the sieving process. _______________________________________________ 53 Fig 41 Planetary ball mill. _________________________________________________________________ 54 Fig 42 Vibratory sieve shaker and sieves. _____________________________________________________ 54 Fig 43 Atmos bag glove Bag with zipper-lock closure by Sigma-Aldrich. ____________________________ 55 Fig 44 Functionalized MWCNTs. ____________________________________________________________ 56 Fig 45 Functionalized Inconel 718. __________________________________________________________ 57 Fig 46 Phase differentiation between Inconel 718 and MWCNTs. _________________________________ 57

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Fig 47 Magnetic Stirrer and hot plate. _______________________________________________________ 58 Fig 48 Ultrasonic Bath. ___________________________________________________________________ 58 Fig 49 Particle size distribution of Semi-spherical morphology. ___________________________________ 59 Fig 50 Particle size distribution of flake morphology ____________________________________________ 59 Fig 51 Semi-Spherical morphology by SEM. ___________________________________________________ 60 Fig 52 Flake morphology by SEM. __________________________________________________________ 61 Fig 53 Semi-spherical morphology with citric acid by SEM. _______________________________________ 62 Fig 54 Semi-spherical morphology with citric acid by XRD. _______________________________________ 62 Fig 55 Semi-spherical morphology with citric acid by FTIR. _______________________________________ 63 Fig 56 Flake morphology with citric acid by SEM. ______________________________________________ 63 Fig 57 Flake morphology with citric acid by FTIR. ______________________________________________ 64 Fig 58 MWCNTs functionalized and seen through SEM. _________________________________________ 65 Fig 59 MWCNTs functionalized and seen through XRD. _________________________________________ 65 Fig 60 MWCNTs functionalized and seen through FTIR. _________________________________________ 66 Fig 61 Semi-spherical morphology with MWNCTs by SEM. _______________________________________ 66 Fig 62 Semi-spherical morphology with MWNCTs by XRD. _______________________________________ 67 Fig 63 Flake morphology with MWNCTs by SEM. ______________________________________________ 67 Fig 64 Flake morphology with MWNCTs by XRD. ______________________________________________ 68

List of tables.

Table 1 Ferrous materials according to the application adapted from [20]. __________________________ 16 Table 2 Types of non-ferrous metals [22]. ____________________________________________________ 16 Table 3 Characteristics of the advanced materials trend adapted from [32]. _________________________ 19 Table 4 Physicochemical properties of Inconel 718 _____________________________________________ 21 Table 5 Chemical composition of Inconel 718 _________________________________________________ 21 Table 6 Experimental design and processing routes. ____________________________________________ 50

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1. Chapter 1: Introduction and State of the art

1.1. Motivation

In the last decades, there has been an exhaustive effort to investigate advanced materials with improved properties as it is mechanical, electrical or thermodynamic performance. An important requirement to develop such advanced materials is that they must be manufactured in a relatively easy way. In the aerospace area, the improvement of existing materials is a part that has a great focus [1]. This is due to the fact that continuous development is needed to avoid major failures in the different parts that make up an aircraft.

It is because of this that advanced materials arise which are a key element in industrial innovation. These are developed as materials with new properties and better performance, which allow to obtain more competitive, safe, efficient products.

The importance of advanced materials in the industrial sector is that they provide production technologies that are aimed at developing cheaper products, with better performance, more durable and with greater added value [2].

These materials have impacted different techniques within the sector and have had an important development in fusion technologies and metal transformation, powder metallurgy, synthesis technologies and advanced ceramics manufacturing., Composite materials of metallic, ceramic and organic matrix, surface technologies and joining technologies. However, there are challenges that they face in the future, which range from improving the performance of current materials to increasingly demanding service actions, reducing the cost of products without reducing their characteristics, more effective new coatings and durable, joining methods between more reliable and versatile materials [3].

Given the implications of these advanced materials, the need to improve existing materials in the different components of an aircraft. Since this gives possibility to create mechanically more stable aircraft in its different phases. Also, in literature many experimental studies explore the possibility of increasing the junction temperature limit, beyond the limits established by manufacturers, in order to increase power capabilities. [4].

These improvements can be made through advanced materials which have been used in parts of the turbine for several years. However, the trend points to something even more viable, which are the nanocomposites, since these being modified from the base structure of the material, leads to raising specific properties that are required [5].

Given the hardness of some metal alloys, it is very difficult to manufacture parts or prototypes, the above has motivated the implementation of new technologies that allow manufacturing more efficiently. It is the reason why material modifications are currently being implemented from their base (nanocomposites) through new

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manufacturing techniques. A clear commercial method of manufacturing alloy metal powders is atomization, in recent years and coinciding with the development of metal matrix composite materials (MMC), processes based on grinding are emerging as an effective method to achieve, both alloys difficult to obtain by other methods such as composite powders [6].

Of all of them, one of the most effective and of most interest is the mechanical alloy.

Mechanization and chemical bonding have proven to be the most favorable technique for obtaining nanocomposites pre-alloyed powders, due to the possibility of a good control of the chemical composition, size, shape and granulometric distribution thereof. These characteristics, important for subsequent compaction, added to the possibility of obtaining a fine microstructure with a high solute content, have made atomization the process of obtaining dust with greater industrial implantation [7].

Against this, the mechanical alloy is the most current mechanical process for obtaining dust, by allowing compositions and levels of properties impossible to achieve by other methods, including atomization. This technique was first studied and developed in the mid-1960s, by John S. Benjamin, which consists of an effort to find a way to combine the hardening by dispersion of oxides with the hardening by precipitation of nickel-based superalloys for applications in gas turbines [8]. Since then, mechanical alloy has diversified its scope from iron-based alloys to aluminum alloys and ceramic systems and is already marketing numerous materials. Defined as a dry grinding process, the fundamental advantage it presents is precisely the possibility of obtaining unattainable compositions by processes such as atomization, raising the solute content well above the equilibrium compositions and increasing the percentage in reinforcements, eliminating the problems of segregation and formation of agglomerates generally associated with conventional powder metallurgical mixtures [9].

The review in the literature indicates that similar methods have been used over time, but that in order to streamline current processes, it is necessary to implement techniques that yield nanocomposites that can be carried through additive manufacturing. These nanocomposites should have as a principle at least stronger chemical interactions that help create an increase in their properties and that as a final result when applied in the industry show a greater ease of being machined [10].

1.2. Ferrous and Non-Ferrous Metals

There are materials that are currently used in different industrial sectors such as:

aerospace, motor, electrical. These materials are classified according to their composition, properties, and applications. But there are two types that especially have enough relevance and multiple applications for many years, these are ferrous and non-ferrous materials [11]. In order to make and understand the study carried out, a thorough investigation of these two types of materials was made and thus be

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able to give a result of why the material to be studied is better or may have a greater range of application.

Starting with non-ferrous materials it is remarkable to say that they are made of iron its alloys, sweet or wrought iron, steel and cast iron. They are the most used due to their low cost of extraction and obtaining. Iron is the chemical element (Fe) that constitutes 5% of the earth’s crust (2^nd most abundant metal). It is not present in its pure state but in combination with other elements in minerals: magnetite, siderite and hematite. Its main drawback is that it can oxidize by reacting with oxygen from the air or water by degrading the metal until it causes it to break [12].

The distinctive attribute of iron is that it is dense, strong when mixed with carbon, abundant and easy to refine, highly susceptible to corrosion and magnetic. Making iron alloy with other elements in different proportions can mitigate or eliminate one or more of these factors [13].

Ferrous alloys with carbon are usually called iron or steel, and can contain any number of other elements, from aluminum to vanadium, based on their specifications. These metals are usually chosen for their mechanical properties.

Multiple investigations are focused on its elastic limit, hardness, ductility, weldability, elasticity, ease of cutting and thermal expansion, which describe how a material will behave under specific stressors. In ferrous metals, iron and steel are the most common foundry materials [14].

1.2.1 Types of Ferrous Metals

It is necessary to emphasize that there are different types of ferrous materials, some with better properties over others, this depends on what application is given. A notable example is cast iron which is a category of iron alloys with carbon content greater than 2%. They are relatively cheap and dense irons. When they are heated and melted, they have much greater fluidity at lower temperatures than steel, this means that they can flow inwards and fill the parts of a complex mold with greater efficiency. Cast iron also shrinks at half the rate of steel while cooling [15].

The applications of ferrous metals are multiple, starting with the applications of iron, which in itself does not have many applications, these can be highlighted:

electromagnets and as a starting material for moldable cast iron, since that is a soft material. Ferrous materials have a lot of uses, they can be differentiated between them: in the construction it is used in a multitude of elements from rods, bars to palastros, beams and slabs; in the shipping industry they are used for their characteristics of armed and salinity resistance, in some of their variations; military industry, its main use in this field are weapons for their resistance to high temperatures and the environment; In the automobile industry, it was used in the seventies, today its use in this field is almost nil and in the food industry stainless steel is used in the manufacture of machinery for its resistance to liquids and for the absence of microbes on the smooth surface of this material [16].

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Foundries are mainly used in the manufacture of parts because they are easy to machine and mold. They are very resistant to mechanical stress and good compression properties, but they are fragile: they will break before bending or distorting. This vulnerability may mean that the fragile grades of cast iron are not used for designs with extruded or elaborated details, or with sharp edges, since these characteristics can be chopped [17]. That is why it is very important to know the microstructure of the ferrous material that is had, since this will determine many of the intrinsic properties of the material.

These challenges mean that steel can be much more intense than working to mold it well. He requires expert attention during all stages of his production. Even so, the high mechanical strength of the final product can make a steel alloy the clear choice for some applications, with the machining providing final stage finishes [18].

Pure iron has very few technical applications because its properties are very poor.

But combine it with small amounts of carbon, a nonmetal, greatly improves its properties depending on the amount of carbon added to iron. For example, it is notorious to distinguish the following alloy: mild iron, steels, and smelters [19]. A clear example is the ferrous of what an alloy of this type looks like, which can be a starting point for determining different mechanical, chemical and metal composition properties is the microstructure of G10100 steel sheet (Figure 1).

Fig 1 Ferrous Microstructure example. Adapted from [20]

There is also another way to classify this type of ferrous materials, this is due to the adhesion of carbon, since the adhesion of this provides properties to the material.

Due to this, the range of application may increase, this is expressed in table 1.

Material Characteristic Properties Applications

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Wrought iron Carbon content less than 0.03%.

·It oxidizes easily and cracks internally.

·Soft, the purer.

·Ductile and malleable.

·Good conductor of electricity.

·Supports the forge to mold it.

·Difficult welding.

Electrical applications.

Electromagnets cores.

Steels Carbon content between 0.03% - 1.76%.

·Excellent mechanical properties:

hardness,

toughness, stress resistance.

·The harder the more carbon.

Under cost of obtaining.

·They oxidize easily.

·They allow good welding.

·They admit forging and machining.

Tools, nails, screws, watches, rail vehicles, cars, boats, beam profiles.

Machinery, cutting, surgical equipment, space vehicles, nuclear reactors

Foundries Carbon content between 1.76% - 6.67%

·Less ductile and less stubborn than steels.

·Bad weld.

·Harder than steels since carbon gives it hardness but increases fragility.

·Easier to machine than steels.

Lampposts, benches, fountains, sewer covers.

Benches or blocks of engines, machinery, boilers, ingots to obtain steel.

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·They melt easily, the lower

temperature than steels and pure iron (400ºC less).

·They allow

obtaining pieces of different

complexity and size using molds.

Table 1 Ferrous materials according to the application adapted from [21].

1.2.2 Types of non-ferrous Metals

Comparing ferrous materials with non-ferrous materials, it can be seen that non- ferrous materials, their applications are more specific, however, since they are non- ferrous materials, it does not mean that there is no iron in their composition, it may be existing but it is not their majority base. Taking into account the most used, and with more range was applied, the most important are 7: Copper, Zinc, Lead, Tin, Aluminum, Nickel and Manganese. There are many other non-ferrous metals;

However, since the quantities produced are small, or the processes are highly expensive, they are rarely used [22]. They are classified according to their density in heavy, light and ultralight. Table 2 shows the types, their characteristics and the existing metals.

Types Characteristics Metals

Heavy Its density is equal to or

greater than 5 kg / dm3 Tin, Copper, Zinc, Lead, Chrome, Nickel, Tungsten or Tungsten and Cobalt.

Light Its density is between 2 and 5 kg / dm3

Aluminum, Titanium

Ultralight Its density is less than 2 kg / dm3

Magnesium

Table 2 Types of non-ferrous metals [23].

Each of these classifications have different applications, however, there are alloys between these materials that can give a higher range of properties and applications.

But if we take into account non-ferrous materials alone, they have characteristics such as: Soft, high corrosion resistance (higher than ferrous alloys, high electrical

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and thermal conductivity, low density, ease of production. That’s why non-ferrous materials are used in manufacturing as complementary elements of ferrous metals, they are also very useful as pure or alloyed materials which, due to their physical and engineering properties, cover certain requirements or working conditions, for example bronze (copper- tin) and brass (copper-zinc) [24].

Therefore, to improve their properties, they are usually alloyed with many more metals. For example, the alloy of Inconel 718, which being an alloy reinforces its properties. Looking at the microstructure (Figure 2), It is possible to know which phases exist mostly, which will give them such unique properties.

Fig 2 Microstructure of Inconel 718

Even when the characteristics are marked in this way, it is not possible to generalize, because each of them can have a completely different property and application. An example of them is aluminum which is a silvery and shiny white metal in a state of high purity, it is quite ductile and malleable compared to its weight it is quite resistant mechanically, it has high thermal conductivity. In its natural state it is found in many silicates (feldspar, plagioclase and micas) [25].

But if we look at another type of non-ferrous material that is highly used, it encounters copper which is a red metal, very malleable ductile state of high purity, has a high thermal and electrical conductivity, copper in normal atmospheric conditions is quite resistant to corrosion, but when the temperature is wet it is discovered with a layer Green jade platinum or cardinal that protects them from further attacks.

Due to its high electrical conductivity, ductility and malleability, it has become the most used material to manufacture electrical cables and other electrical and electronic components [26].

Placing one of the non-ferrous materials used strongly in the aerospace sector, is the nickel and its alloys, this is because the nickel is hard and very stubborn when it contains a small amount of Carbon, it becomes malleable, leaving small amounts of magnesium laminated, very resistant to atmospheric corrosion, and allied to iron it

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imparts great oxidation resistance. These alloys are used to make coins, jewelry, and items such as valves and heat exchangers. Most of the nickel is used to make stainless steel [27].

It can also be combined with other elements, such as Chlorine, Sulfur and Oxygen to form nickel compounds. Many nickel compounds dissolve easily in water and are green in color. Nickel compounds are used in nickel plating, to color ceramics, to make batteries and as catalysts, which are substances that accelerate chemical reactions [28].

Highlighting ferrous and non-ferrous materials as materials that are already used today, it is necessary to know where they are going and how they plan to apply in the future. This is why advanced materials arise, which are conjunctions of ferrous and non-ferrous materials.

1.2.3 Trend of advanced materials

Advanced materials are found within the entire spectrum of types of materials and can be classified as: metallic materials, high performance polymers, advanced ceramic materials, composites and biomaterials. These materials show superior mechanical characteristics of strength, hardness, toughness and durability in relation to conventional materials [29].

The advanced materials production technologies are aimed at developing cheaper products, with better performance, more durable and with greater added value.

OBJECTIVES – CHARACTERISTICS EXAMPLES OF DEVELOPED PRODUCTS

Superior mechanical properties, including toughness, fatigue resistance and wear resistance.

Low alloy steels, economical, with very high strength and deformability, to increase the safety of cars and transport in general.

Use of durable coatings to achieve the required surface properties without altering the characteristics of the original base material.

Light alloys and composite materials for the automotive and aeronautical sectors.

Specific surface treatments (chemical and mechanical) to increase the performance of the products and maximize their durability.

Alloys and coatings resistant to high temperatures, to the action of very aggressive environments and with a greater tolerance to damage.

Complex on-demand microstructure design to maximize the performance of industrial components.

New steels for higher performance rail, which ensure greater durability before increasing service loads.

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Materials to work in extreme environments (high and low temperatures, very corrosive environments).

Tenacious ceramics capable of supporting with sufficient reliability mechanical and thermal loads.

Materials with new functionalities and better performance throughout its useful life.

New coatings resistant to corrosion and wear.

Light and sustainable materials for the automotive industry and for the aerospace industry.

New materials easy to be machined and still with high mechanical properties.

Table 3 Characteristics of the advanced materials trend adapted from [29].

Some of the challenges that will be taken for the future is to be able to provide materials that increase the parameters that the system has. A clear example is aerospace ships, which could be faster if the composite material in its turbine withstood a higher temperature.

This is something that is already being carried out in experimentation phases, a sample of this are the advanced compounds based on titanium / aluminum and carbon fiber, which generate a material 3 times more resistant to steel, maintaining at a time, a considerable support for high temperatures [30].

One of the materials that is rising in its applications and that are being considered strongly are super alloys, since they show excellent mechanical and creep resistance (tendency of solids to move slowly or deform under tension), resistance to high temperatures, stability and high resistance to corrosion and oxidation. Much of these properties are the nickel super alloy, the so-called Inconel 718.

1.2.4 Super alloy Inconel 718

The Inconel 718 is a material that began to be developed in the mid-twentieth century and used profusely since the 60s for the manufacture of aeronautical engine components among other things, and that is still used today in many reactor components. This is not an obstacle for further research into the evolution of this alloy by the material departments of companies in the sector or the introduction of new ones with improved properties [31].

Nickel-based alloys operate in the components of an aeronautical gas turbine at temperatures as high as 80-85% of their melting temperature, which makes it necessary in certain areas to use aids such as internal cooling of the blades themselves by means of micro holes arranged for this purpose, or the use of ceramic thermal barriers to allow a high operating temperature and thus maintain the efficiency of the motors. This thermodynamic efficiency of the thrusters is, as is known, the higher the higher its operating temperature, and at this point the current state of the art meets the maximum physical temperature limit bearable by the materials of the components that integrate the motor [32].

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This is the trend of current research in this type of materials, to find the right key to achieve a new superalloy that allows maintaining the level of properties of mechanical resistance, corrosion resistance, creep resistance. at a higher operating temperature of the components.

One of the great challenges in the metalworking industry is to get the perfect, valid tool to machine any material that comes your way. This impossible today, does not prevent the tooling industry from being a constantly evolving industry, to which new challenges are presented every day in the form of new materials to be machined or evolutions of those that already existed [33]. With a lot of security in the metalworking industry, the families of the most demanding materials for machining are super alloys. These characteristics may be related to its poor thermal conductivity, chemical affinity with the tool, ductility, or presence of abrasive particles, for example, which constitute important threats to perform quality machining.

Among these materials are super alloys, which have had a high impact of applications in the automotive, aerospace, metallurgical and environments where properties exceed existing alloys. Among the most applicable and with good properties is the super alloy Inconel 718. This is a nickel-base superalloy largely used in the fabrication of critical pieces for turbine engines because of its high mechanical properties (yield strength up to 650ºC, impact strength and fracture toughness down to -40ºC), as well as good corrosion resistance. In addition, this alloy can be easily forged and welded.[34]

Its superior mechanical properties and its resistance to high temperatures and to corrosion have led to its extensively used as a structural material in a wide variety of applications in the aerospace industry, such as disks, blades and shafts in gas turbine engines, where a high level of strength under high-temperature conditions is an important design consideration. [35]

The great impact and application of Inconel 718 is reflected in its physicochemical properties, which can be applicable in different fields and can be modified if a nanocomposite is created. The most important properties of Inconel 718 are shown in table 4.

Property Value

Electrical Electrical Resistivity 125 µOhmcm

Physical Density 8,19 g cm^-3

Melting Point 1533.15- 1608.15 K Magnetic Initial permeability 1,0013

Curie temperature <- 469.15 K

Mechanical Elongation <15 %

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Brinell hardness 250-410 Elasticity Module 200 Gpa Tensile Strength 800-1360 Mpa Thermal Specific Heat at 23C 435 J K^-1 kg^-1

Thermal Expansion

Coefficient

13,0 x10^-6 K^-1 Thermal Conductivity at

23C

11,2 W m^-1 K^-1 Maximum Air Use

Temperature 973.15 K

Table 4 Physicochemical properties of Inconel 718

Inconel alloys belong to the Ni–Cr based superalloys family which cover a wide range of compositions and mechanical properties. Ni and Cr provide resistance to corrosion, oxidation, carburizing and other damage mechanism acting at high temperature. Inconel alloys have good cryogenic properties, good fatigue and mechanical strength at moderate temperatures and relatively good creep behavior.

Usually, Inconel alloys are extra-alloyed with Al, Ti, Nb, Co, Cu and W to increase mechanical and corrosion resistance. Fe can also be present in amounts ranging 1–

20%. These superalloys are indented for heat treatment recipients, turbines, aviation, nuclear power plants, and so on. [36] The Inconel 718 is composed of the elements described in table 2 [34].

The strengthening phase in the alloy 718 is the metastable body centered tetragonal 𝑁𝑖_*𝑁𝑏 (𝐷𝑂_--) 𝛾^// phase [34] (shown in Fig 3).

Element Percentage

Ni 53

Cr 19.06

Fe 18.15

Nb 5.08

Mo 3.04

Ti 0.93

Al 0.52

Co 0.11

C 0.021

Cu 0.02

Table 5 Chemical composition of Inconel 718

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Fig 3 Body Centered Tetragonal atomic arrangement.

Also, the existence of a small amount of a 𝛾^/ phase 𝑁𝑖_*(Ti, Al) (shown in figure 4) (fcc 𝐿1_- structure), appearing as a fine dispersion, of quasi-spherical particles which are coherent with the matrix. The role of the 𝛾^/ phase is also strengthening, but to a lesser degree than the Inconel 718 is a nickel base superalloy largely used in 𝛾^/′ precipitates (the volume fraction of 𝛾^/′ is four times the manufacture of critical pieces for turbine engines be- bigger than that of 𝛾^/) [37].

Fig 4 Face Center Face atomic arrangement

Taking into account its phases, properties and advantages, it is said that the family of superalloys constitutes one of the most demanding in terms of its metallurgy.

These materials are generally Nickel-based alloys and have widespread use in the aeronautical sector due to their exceptional mechanical strength and toughness characteristics, as well as high corrosion resistance, all at high temperature.

However, it is one of those that present a more difficult machining. This is why in recent years there have been numerous advances in the field of superalloys, both in terms of metallurgy and processing methods, especially in relation to its machining, both in techniques and tools [38].

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These advances are the logical consequence of investment and continuous research in this type of materials by aircraft engines and smelters. Traditionally, a multitude of different components have been manufactured with this alloy, mainly for the hot zones of aeronautical engines, for their unmatched properties already mentioned, such as discs, housings, seals, spans, blades, etc. and until recent times there has not been much variation in this aspect [39]. But the new efficiency requirements of the new generation of aeronautical engines make the materials have to be able to withstand higher temperatures in service and therefore a new generation of materials with improved properties is necessary to allow these necessary increases in operating temperature [40]. That is why it is necessary to propose an improvement of Inconel 718 through a reinforcement, since the technologies currently applied do not allow to exceed that limit which would allow a wide range of application.

Currently, Inconel's approach to adhesion of carbon nanotube reinforcements throws the additive manufacturing (AM) this technology develops rapidly in the recent years, which has shown a large potential and annually growing market of manufacturing [41]. Among all the AM techniques, electron beam melting (EBM), selective laser melting (SLM) and selective laser sintering (SLS) are the most widely used AM processes, which can manufacture near-net-shape parts directly up to 99.9% relative density [42].

While it is necessary to provide a substantial amount of depositing powder in all of the powder bed AM methods mentioned above. To overcome this deficiency, laser powder deposition (LPD) is applied, which is also termed as laser metal deposition (LMD), laser cladding or laser deposition. Compared with the powder bed AM processes, LPD consumes a much lower amount of powder and in turn, provides much greater system flexibility to produce the additive layers as the depositing powder is fed and melted into the molten pool simultaneously through a coaxial or lateral nozzle during the deposition process. Therefore, it is proved to be more feasible for repair or remanufacturing, especially for the repair of high-valued nickel- base superalloy components [43,44].

1.2.4.1 CNTs in a Metal Matrix

The properties in CNTs-Metal compounds often deteriorate seriously with the increase in CNT content, especially when the CNT content is greater than 2% by volume, this problem is attributed to three reasons: first, it is very difficult to homogenize the dispersion of large quantities of CNT In the metal matrix , the strong interactions of Vander Waals generate agglomerations of CNT. Secondly, the specific surface area of CNT is so large that the interface / volume ratio is much larger than that of conventional matrix compounds, so that the properties are more sensitive to the interfacial bond, thirdly, porosity after consolidation and sintering [45].

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It’s important to develop effective manufacturing methods that not only can disperse high-content CNT in the Inconel matrix, but can also achieve a good bond between the CNT and the metal matrix. Flake powder metallurgy has many advantages in disperse a high content of nano reinforcements evenly into metal nanoflakes.

The disk-shaped nano scales with dozens of microns in diameter and a few hundred nanometers thick have a greater CNT dispersal capacity than metal sphere particles.

Also, there are a better dispersion of CNT by surface modification in both Metal matrix particles and carbon nanotubes, this functionalization of the surface leads to a strong bond and prevents regrouping during filtering and drying [46].

1.2.4.2 Areas of opportunity

To achieve successful manufacture of the nanocomposite, there are several barriers or areas of opportunity to overcome. We need conductive carbon nanotubes connected each other embedded in a metal matrix in a homogeneous way, also a metal matrix properly designed to maximize the good adhesion between the matrix and the nanotubes is required, [47]. Additionally, stable productions methods are required which prove nanotubes homogeneously dispersed in the matrix. Similarly, the cost of production must be low to allow the use of large quantities of carbon nanotubes [48].

Traditionally, a multitude of different components have been manufactured with Inconel 718, mainly for the hot zones of aeronautical engines, for their unmatched properties already mentioned, such as discs, housings, seals, spans and blades.

And until recent times there has not been much variation in this aspect. But the new efficiency requirements of the new generation of aeronautical engines make the materials have to be able to withstand higher temperatures in service and therefore a new generation of materials with improved properties is necessary to allow these necessary increases in operating temperature [49].

Fig 5 Internal parts of a turbine.

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1.3 Carbon allotropes

The different carbon phases have been used for many years to add properties to metals and alloys, which without this adhesion would not be possible to have them.

That is why it is very important to be able to study in depth the existing allotropies, since each one can add advantages or disadvantages when being added to metallic matrices. There are different carbon allotropes such as fullerenes, graphenes, carbon nanotubes have been investigated strongly lately [50].

1.3.1 Fullerenes

Fullerenes have had utility with metals, but mostly to encapsulation of them. That is why they took place in modern materials for nanoscience, as far as carbon-based materials are concerned. Since its discovery, fullerenes have been the focus of attention for the scientific community. The fullerene 𝐶₃₄ is the most abundant and representative [51]. In the 18^th century, the Swiss mathematician Leonhard Euler showed that a geodesic structure can contain 12 pentagons to become a spheroid, although the number of hexagons can vary. Fullerenes are represented by the expression 𝐶₅, where n represents the number of carbon atoms present in fullerene.

Fullerenes are formed with an even number n ≥ 20 of three vertices, 3/2 edges and (- 20) / 2 hexagonal faces [52]. The icosahedral forms of the fullerenes lead to a concentration of curvature [53] (or pyramidalization in the carbon atoms) in the five- member rings, which begin to assume the role of vertexes in the fullerene in a polyhedral form [54]. This observation has implications for the reactivity of the

fullerenes [55].

The pyramidalization angle 𝜃𝑝 is defined as 𝜃𝑝 = 𝜃𝜎𝜋 − 90º, where 𝜃𝜎𝜋 is the angle between the 𝜋 orbital and the 𝜎 bond of the C atom of interest [51]. For a carbon atom with hybridization 𝑠𝑝^- (trigonal), a planar structure is strongly preferred, this implies a pyramidalization angle 𝜃_A = 0º, whereas a hybridization 𝑠𝑝^* (tetrahedral) carbon atom requires 𝜃_A = 19.5º (Figure 5). The atoms of 𝐶₃₄ have 𝜃_A = 11.6º, it is clear that their most appropriate geometry would be Tetrahedral [56].

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Fig 6 Pyramidalization angles present in different molecular structures [51].

This pyramidalization has been taken into account because it is a similar process that occurs in graphene sheets, forming forms such as SWCNTs. It is through this process that the use of fullerenes is in mind as a good possibility of being applied to metals not only encapsulating them, but also being able to play an important role in surface adhesion.

1.3.2 Graphenes

The technology industry has placed much of its hopes in graphene, because due to its structure and reactivity, when added to a metal matrix, graphene sheets play an important role in increasing their properties. This is because the graphene is a structure of carbon atoms that form a regular hexagonal pattern that are generated from hybrid 𝑠𝑝^- orbitals on carbon. The detailed reactivity of graphene sheets, in terms of size, shape and the possibility of a stoichiometric control, is still explored [57]. However, it can be expected that the royal selectivity based on zig-zag or arm- chair at the terminations in the graphene plane will manifest certain reactions. The formation of a covalent bond in the basal plane of the graphene sheet requires the breaking of 𝑠𝑝^- bonds and formation of 𝑠𝑝^* bonds [58]. An unpaired electron that is created in the place adjacent to the covalent attachment point improves the reactivity there, which leads to a chain reaction from the initial point of attack.

In addition, the principle of geometric stress minimization can establish reactivity.

Figure 6 illustrates that the covalent bond between a hydrogen and carbon atom will cause atomic displacement of bonded carbon above the plane of the sheet, and this stress can be minimized by having a second hydrogen atom anchored to the opposite side of the leaf releasing tension.

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Fig 7 Graphene leaves a) Pristina, b) Hydrogenated and c) Stressed

The geometrically tense regions in the graphene network are areas of preferential reactivity due to the displacement of electrons above the plane. Due to the 2D nature of the graphene sheets, the edge regions play an important role in the electronic structure of the molecules. The edges can be manifested as a zig-zag or arm-chair.

At the zigzag edges, the attainment of aromatic sextets is frustrated in most rings, therefore a structure of this type is thermodynamically unstable in comparison with the armchairs. It is then expected that the zigzag edges show greater reactivity compared to the armchairs [59].

This geometry is strongly used as an advantage when making an adhesion with some other material, since due to this versatility in its geometry it is possible to have different shapes within the same matrix, one transposed above another. This helps in principle that there is a greater connection between graphene sheet that speaks of a greater increase in electrical conductivity for example.

1.3.3 Carbon Nanotubes

The great limitation of carbon nanotubes (CNT) in practical applications is the great difficulty of manufacturing devices in which they are placed in a controlled and well aligned way. Today, several thousand tons of CNT are manufactured annually, but their utility is reduced to applications where they can be placed randomly and in large quantities.

As the synthesis, purification and chemical modification techniques of the CNT have advanced, they have been incorporated into thin film electronics and surface coatings. In these applications films are used in which the CNTs are messy, such as thread forests. The mechanical, thermal, and electrical properties of these CNT macrostructures are much lower than those of individual CNTs. But manipulating CNTs one by one at the industrial level is beyond current techniques.

This is due to the structure of the CNTS since it can be considered as a sheet of graphene rolled on itself [60]. Depending on the degree of winding, and the way in which the original sheet is formed, the result can lead to nanotubes of different diameter and internal geometry. These tubes shaped as if the ends of a paper were

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joined at their ends forming a cylinder are called monolayer nanotubes. There are also nanotubes whose structure resembles that of a series of concentric tubes, including some within each other, of increasing thicknesses from the center to their surroundings. The latter are multilayer nanotubes. Generally, monolayer nanotubes have a diameter of approximately 0.7 to 10 nm and a length of the order of microns, so their ratio length / diameter is approximately 10^D− 10^E and due to this is that they are considered one-dimensional structures (1D) [61].

1.3.4 Single-wall Carbon Nanotubes

In the industrial sector, the adhesion of SWCNTs in metal matrices for the reinforcement of metal has been experienced. However, being able to add SWCNTs is a complicated process, since producing them is only highly expensive. That combined with SWCNTs are very volatile, which makes them difficult to be handled and industrialized. When talking about them stably, this is an important factor since producing defects in them for a functionalization is more difficult and expensive because of their intrinsic stability.

Since these are formed of a cylinder and generated when a graphene sheet of a certain size is wrapped in a certain direction in such a way that two crystallographically equivalent sites of the hexagonal network coincide. An SWNT consists of two different regions with different physical and chemical properties. The first region is the graphene sheet that forms the wall of the tube and the second region is the lid (carbon atoms C placed in hexagons and pentagons form the structure) that covers the end of the tube. The cap of the tube sometimes has a structure similar to that of fullerene 𝐶₃₄, so that if the two caps were joined, they would generate a 𝐶₃₄ molecule or another fullerene type [62].

1.3.5 Double-Wall Carbon Nanotubes

If SWCNTs turn out to be expensive to be applied in the industrial sector, the focus is on nanotubes with more walls. This is where the Double-wall Carbon Nanotubes (DWCNTs) come in, which, being double-walled, maintain a lower stability, making them more manageable, more optimal to be able to develop processes such as functionalization or simple adhesion. It is important to note that DWCNTs consist of two individual carbon nanotubes, with one embedded inside the other.

The differences in diameters and the chirality of the two different nanotubes lead to a varying degree of interaction between the two, while at the same time the properties of the individual nanotubes being separate from each other [63].

It is this wide variety of possibilities that have made DWCNTs a focus of interest for carbon nanotube research. Varying chirality allows a range of inner-wall outer-wall interactions to occur, because the chirality determines whether the nanotube will be semiconducting or metallic. It is possible to achieve metallic-metallic, semiconducting-metallic, metallic-semiconducting or semiconducting- semiconducting interactions. In addition to this, the metallic and semiconducting

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properties can vary depending upon the exact lattice parameters, which enables a wide range of possible property combinations [64].

DWCNTs also have a large advantage over single-walled carbon nanotubes, as it is possible to modify the outer nanotube without changing the properties of the inner nanotube. This modification could be either through functionalization (to add solubilizing groups), or the doping of the structure (to alter the properties). This allows the double-walled system to maintain functionality of a single-walled nanotube whilst simultaneously having the solubility of functionalized nanotubes.

1.3.6 Multi-wall Carbon Nanotubes

One of the most used in the industrial sector is the Multiwalled carbon nanotubes, this is because its multiple walls allow the nanotubes to have an instability that is used as an advantage so that they can be processed by different routes. Having multiple capable, contrary to what one might think, these are cheaper and more accessible to handle. It is the reason why they are taken into account to be added in different metal alloys to increase some of their properties. Having this as a precedent, Multiwalled carbon nanotubes are considered as a novel nanocarbon material, have been drawing increasing attention recently [65].

This type of carbon nanotubes (CNTs) is close to hollow graphite fibers, except that they have a much higher degree of structural perfection. Their graphite-like structure tube-wall, the nanometer-sized channel, the 𝑠𝑝^- carbon-constructed surface, and the excel lent performance of hydrogen adsorption and electron transport, make them promising in such applications as storage of hydrogen and some other gases, polymer additives and catalyst support material, adsorption–sepa ration medium, efficient field emitters, and electronic components (high-power electrochemical capacitors, high surface-area electrodes, etc.) [66].

Fig 8 Representation of MWCNTs.

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1.4 MWCNTs/Inconel 718 Nanocomposite

If the adhesion of the MWCNTs is linked to an increase in properties, it is highly desirable to go beyond what is already done and these are the nanocomposites which are high performance materials that exhibit combinations of unusual properties and unique design possibilities. They are formed by the union of two or more materials, in order to achieve the combination of mechanical and conductive properties. The properties of nanocomposites do not depend only on individual patterns, but also on their morphology.

Nanocomposites are high performance materials that exhibit combinations of unusual properties and unique design possibilities. With an estimated annual growth rate of 25%, its potential is so prominent that they are applicable in many areas [67].

Metal nanocomposite can also be known as reinforced compounds. Some of their functions are the protect the fibers or particles from the outside environment, the promote the union of the elements that constitute the reinforcement and give good resistance to corrosion or oxidation [68].

There are not many studies made about to develop Inconel 718 matrix compounds reinforced with carbon nanotubes. But there are different with metals matrix, resulting good thermal and electrical conductivity, high tensile strength to weight ratio, ductility and hardness [69], [70]. Many methods have been developed to manufacture MWCNTs-Metal compounds, such as cumulative rolling bonding, high energy ball mill [71], friction stir processing and powder metallurgy on scales [72].

Nanocomposites of MWCNTs in Inconel has been carried out through different processes such as adhesion of MWCNT arrays synthesized directly on nickel superalloy substrates employing a vapor-phase catalyst delivery method in a CVD chamber. [73] In this case, the carbon nanotube growth was carried out via floating catalyst thermal chemical vapor deposition using xylene as the main carbon source and ferrocene as a catalyst.

Another method of adhesion of MWCNTs is through Selective laser melting (SLM).

Which is used for additive manufacturing. The MWCNT are coated with Inconel powder by mixing at the molecular level. Compared to the unprocessed Inconel sample, the results show a significant improvement in tensile strength and the final limit with less elongation. Also, with respect to the heat treatment in the composite sample it recovers its elongation to the level of pure Inconel while maintaining its maximum tensile strength but reducing its elastic limit [74].

The nanocomposites that are based on the adhesion of MWCNTs in matrices such as that of Inconel 718 also show innovation not only in their basic structure, but also thought that once created, it can have an improvement in the industry. An example of this is the adhesion of MWCNTs as nanofluids, which has been shown that once added the machinability is improved when processed, this leads us to consider that the adhesion of MWCNTs is not only intended for the basic process, but for an industrialized process [75].

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1.4.3 CNTs oxidation

The adhesion of the CNTs to metal matrices is carried out through different processes where the dispersion of the CNTs is not taken into account. But if it were possible to distribute CNTs homogeneously in the matrix of any metal or alloy, it would be through a chemical interaction between the CNTs and the matrix. That is why this is thought of an oxidation of the surface walls of the CNTs that could lead to a good interaction if the matrix to which it will be added, also has affinity with the CNTs. This reactivity of organic compounds can be attributed to atoms or group of specific atoms within the molecule. A site of reactivity in an organic molecule is known as a functional group because it controls the behavior of the molecule. Thus, the chemistry of an organic molecule is largely determined by the functional groups it contains [76]. Chemical modification in carbon nanostructures is an area of study in materials science.

The most common methods used to carry out the functionalization are:

• Covalent Functionalization

• Non-Covalent Functionalization 2.1.4.1 Covalent Functionalization of CNTs

The process of covalent functionalization in carbon nanotubes has been studied from the point of view of carbon-carbon double-bond (alkene) reactions. The oxidation of alkenes can carry out the formation of functional groups by different types of reactions [77].

Oxidation of alkenes

Carboxylic acids can be obtained by breaking up alkenes with potassium permanganate in acidic or basic media and heating. This reaction generates products similar to ozonolysis, although instead of aldehydes it gives carboxylic acids (Figure 9).

Fig 9 Oxidation of alkenes by potassium permanganate (KmnO4).

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We can also obtain similar results from the oxidation of alkenes with ozone followed by treatment with hydrogen peroxide (Figure 10).

Fig 10 Oxidation of alkenes by ozone followed by hydrogen peroxide.

On the other hand, the hydration of alkenes is carried out with water. Water is a very weak acid, with insufficient proton concentration to initiate the electrophilic addition reaction. It is necessary to add an acid (H2SO4) to the medium for the reaction to take place. This reaction generates alcohols (Figure 11).

Fig 11 Oxidation of alkenes by hydration of alkenes.

Covalent functionalization in the walls is associated with a change in the hybridization of 𝑠𝑝^- to 𝑠𝑝^*. The fluoridation of carbon nanotubes has been the subject of research for the onset of covalent functionalization because the side walls of the CNTs are inert [78], [79]. Fluorinated carbon nanotubes have weak C-F bonds (Figure 12) compared to alkyl fluorides [80] and provide substitution sites for further functionalization [81]. The replacement of fluorine atoms with amino, hydroxyl and alkyl groups has been shown.

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Fig 12 Formation of covalent bonds C-F.

Another method is the defects of carbon nanotubes. These defects are intrinsic and can be used to carry out oxidative damage to the structure of nanotubes by strong acids that leave holes functionalized with oxygenated functional groups [82]. In particular, treatment of CNTs with strong acid such as nitric acid (HNO3), sulfuric acid (H2SO4) or a mixture of them [83] or with strong oxidants such as potassium permanganate (KmnO4) [84], ozone [85], plasma reagent [86] tend to open these tubes to subsequently generate oxygenated functional groups such as carboxylic acid, ketone, alcohol and ester groups, which serve to anchor many different types of chemicals to the ends and defect sites of these nanotubes (figure 13). The chemical reactivity presented by these functional groups in carbon nanotubes can be used as precursors for additional chemical reactions, such as silanation, esterification, thiolation [85].

Figure 1.13. Scheme of chemical modification of carbon nanotubes by acid treatment for anch

1.4.4 Metal-Matrix Functionalization

Considering the adhesion of the reinforcement phase, it is necessary to prepare the metallic phase, which will be processed by a functionalization in which carboxyl groups will be added on the surface, which will have a chemical affinity with the reinforcement phase, in this case, the MWCNTs. There are different ways to functionalize a metal matrix, however, the metal surface must be taken into account.

Fig 13 Scheme of chemical modification of carbon nanotubes by acid treatment for anchoring carboxyl groups – adapted from [86].

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This is because from there it will be defined what will be the element that will give way to functionalization.

A very promising ally for the functionalization of Inconel 718 is citric acid which is a widely used surface-modifying ligand for growth and processing of a variety of nanoparticles; however, the inability to easily prepare derivatives of this molecule has restricted the development of versatile chemistries for nanoparticle surface functionalization. The synthesis of a derivative such as citric acid which has an alkyne group demonstrates that this molecule provides the ability to achieve stable, multi-carboxylate binding to metal oxide nanoparticles [87]. The broad utility of this strategy for the modular functionalization of metal oxide surfaces demonstrated its application in the modification.

Highlighting this assertion, the procedure proposed by S. Riaz which establishes that through a sol-gel process, the surface of Fe3O4 nanoparticles can be functionalized [88]. For making these nanoparticles useful in MRI contrast agent, nanoparticles were coated with citric acid. Citric acid (C6H8O7) is a short-chained molecule containing three carboxylate groups [89,90]. Citric acid gets adsorbed on the surface of superparamagnetic iron oxide nanoparticles (SPION) coordinating through one or two carboxylic group therefore leaving at least one carboxylic acid group unattached.

This not only provides additional functionality but also prevents the agglomeration and oxidation of nanoparticles [91].

Fig 14 Fe3O4 coated with citric acid – Adapted from [88].

Citric acid also proves to be a good carboxylic acid capable of coating different oxidative surfaces such as ZnO, Fe2O3, TiO2, and WO3. This is stated by Lee M.

Bishop, who uses citric acid on these metal oxides, making a kind of functionalization. This in order to leave the particles ready for later processes where the particle needs to have free carboxyl groups that serve as binders for some other adhesion [92]. The process of adhesion of citric acid to metal oxide is shown in figure 15.

1. Chapter 1: Introduction and State of the art

Table of Contents

1. Chapter 1: Introduction and State of the art